Continuously processing waste lignin into high-value carbon nanotube fibers

High value utilization of renewable biomass materials is of great significance to the sustainable development of human beings. For example, because biomass contains large amounts of carbon, they are ideal candidates for the preparation of carbon nanotube fibers. However, continuous preparation of such fibers using biomass as carbon source remains a huge challenge due to the complex chemical structure of the precursors. Here, we realize continuous preparation of high-performance carbon nanotube fibers from lignin by solvent dispersion, high-temperature pyrolysis, catalytic synthesis, and assembly. The fibers exhibit a tensile strength of 1.33 GPa and an electrical conductivity of 1.19 × 105 S m−1, superior to that of most biomass-derived carbon materials to date. More importantly, we achieve continuous production rate of 120 m h−1. Our preparation method is extendable to other biomass materials and will greatly promote the high value application of biomass in a wide range of fields.

In this manuscript by Liu and co-workers, the authors investigated the possibility of synthesis of carbon nanotubes from kraft lignin solution in methanol. Based on this concept, a direct spinning variant of CVD was utilized to manufacture fibers in a continuous manner. The results are interesting, but some issues should be addressed first before the submission can be reconsidered for publication in Nature Communications. Please find suggestions below: 1) "The carbon sources used in this method are mainly from petroleum fine chemicals, such as methane, ethylene, ethanol and xylene" -aromatic solvents such as toluene should also be mentioned due to their widespread use 2) "After post-treatment, the lignin-based CNT fibers were endowed with a tensile strength of 1.35 GPa and an electrical conductivity of 6.28×105 S m-1 . In addition, the continuous production of CNTs fibers from lignin with a 120 m h-1 production rate was achieved." -these values should be compared with the whole state of the art (not just narrowed down to CNTs synthesized from natural resources but also from synthetic precursors such as alkanes and aromatic hydrocarbons). Such a summary would be useful to evaluate how good the reported values really are 3) "TGA result shows that the mass fraction of the CNTs in the aggregates is 75.3% (Figure 3h), which indicates that the lignin-based CNT fibers have high purity" -judging by the provided thermogram, the nanotubes are of poor crystallinity. Yet, the authors report extremely high electrical conductivity and thermal conductivity. Please comment on this issue. 4) Regarding the electrical conductivity of CNT fibers, a primary source of error, which may greatly affect the result, is the cross-section area. Because the authors report very high electrical conductivity values (on the order of thousands of S/cm), more information should be provided on how these values were obtained (especially how the diameter was established). Currently, the following description is not very informative "The determination of electrical conductivity was performed on a Digit Graphical Touchscreen Digital Multimeter (DMM6500 6½)". Was it a two-or four-probe approach? 5) Whenever possible, error analysis should be conducted. The absence of error bars casts doubt about the statistical significance of the reported data. 6) Minor comment, in Table S1, it is recommended to change "Layer number" to "CNT type". "MWNT" and "SWNT" are not numerical values. We fully investigated other common processes for preparing CNT fibers, including wetspinning 2,3, array-spinning 4,5 , and aerogel-spinning 6 . For the wet-spun CNT fibers 2,3 , the first step is to dissolve CNTs in chlorosulfonic acid to remove impurities, so the CNT fibers obtained by this method has high purity. However, the preparation process is complicated, and chlorosulfonic acid has environmental pollution problem. For the array-spun CNT fibers 4,5 , Fe is deposited on Si substrate to catalyze CNT synthesis, so the resulting CNT array has almost no Fe impurity. Although this method can obtain ultra-high purity CNT fibers, it is not only complicated, but also cannot achieve continuous preparation of CNT fibers. For the work mentioned by the reviewer (https://doi.org/10.1038/srep03903) 6 , although high purity CNTs (less than 5% impurity) can be obtained by aerogel-spinning, the impurity content of the CNTs is more than 5% or even more than 15% in some conditions, similar to that obtained in our work. In addition, similar CNT purity was obtained by aerogel-spinning in other reported work 7 . It should be noted that it is possible to further improve the purity of CNTs by adjusting the synthesis process using lignin as carbon source.
In order to avoid misunderstanding, we deleted the statement that lignin-based CNT fibers have high purity in the revised manuscript.
Corresponding changes: Page 9 in Manuscript: TGA result shows that the mass fraction of the CNTs in the aggregates is 82.7% (Fig. 3h), which is similar to the purity of the CNTs prepared by the same method 45 . Note that Fe in the sample was converted to Fe2O3 when heated at high temperature in air, so the removal of oxygen in Fe2O3 is required to calculate the impurity content 46 . In addition, based on the carbon content (61.9%) and feeding rate (4.8 mg min -1 ) of lignin as well as the preparation rate (1.46 mg min -1 ) and purity (82.7%) of the CNT aggregates, the yield of the CNTs is about 40.6%.  In our work, we studied the effects of synthesis temperature, thiophene content and solvent dispersion on the morphology of CNTs, as shown in Fig. 3  We also investigated the effect of lignin concentration on the morphology of CNTs. The results show that single-walled carbon nanotubes (SWNT) and double-walled carbon nanotubes (DWNT) can be obtained when the lignin concentration is lower than 0.8 mg mL -1 ( Supplementary Fig. 17). The acquisition of SWNT and DWNT can be attributed to the reduced amount of carbon deposited on the surface of the Fe catalyst. The existence of radial breathing mode (RBM) stretching vibration peak (100-300 cm -1 ) in Raman spectrum also proved the synthesis of SWNT (Supplementary Figs. 17d and 17e). When the lignin concentration is higher than 5.5 mg mL -1 , the excessive lignin concentration will cause too many wall layers of CNTs, and a large number of carbon nanorods and amorphous carbon spheres also can be (a) TEM image, (d) Raman spectrum, (g) growth mechanism of the CNTs prepared with lignin concentration of 0.4 mg mL -1 . (b) TEM image, (e) Raman spectrum, (h) growth mechanism of the CNTs prepared with lignin concentration of 0.8 mg mL -1 . (c) TEM image, (f) Raman spectrum, (i) growth mechanism of the CNTs prepared with lignin concentration of 2.5 mg mL -1 . TEM images of the (j) CNT aggregates prepared with lignin concentration of 5.5 mg mL -1 , and enlarged images of (k) carbon nanorods and (l) amorphous carbon spheres.

9
( Supplementary Fig. 17). The acquisition of SWNT and DWNT can be attributed to the reduced amount of carbon deposited on the surface of the Fe catalyst. The existence of radial breathing mode (RBM) stretching vibration peak (100-300 cm -1 ) in Raman spectrum also proved the synthesis of SWNT (Supplementary Figs. 17d-e). When the lignin concentration is higher than 5.5 mg mL -1 , the excessive lignin will cause too many wall layers of CNTs, and a large number of carbon nanorods and amorphous carbon spheres also can be formed (Supplementary Figs. 17j-l). Figure 3a is required.

Response:
We thank the reviewer for the comments.
According to the suggestion of the reviewer, we added the scale bar in Fig. 3a in the revised manuscript.

Corresponding changes:
Page 11 in Manuscript:  Figure S17c and d were obtained from distorted surfaces/areas and, therefore, should not be used to determine the density of the fiber structure.

Response:
We thank the reviewer for the comments.
In Supplementary Fig. 17 in the original Supplementary Information, we provided the cross-sectional SEM images of the TCFs and RCFs to intuitively compare the internal microstructure of the CNT fibers obtained by two different densification methods, rather than calculate the density of the CNT fibers.

Page 22 in Manuscript:
The density of the CNT fibers is calculated based on their mass and volume.

Comment 6:
The authors mentioned that the process had low energy consumption. How was the energy consumption estimated in this study? This should consider the post-treatment processes.

Response:
We thank the reviewer for the comments.
Since the energy consumption of CNT fibers in the preparation process is mainly concentrated in the high-temperature processing part, so only the energy consumed by lignin pyrolysis was considered in our original manuscript.
For the post-treatment process of CNT fibers, the energy consumption is mainly concentrated in fiber collection, twisting and rolling. An optical axis motor (2GN-18K-50K, rated power=6 W) from Taizhou Weichuang Electromechanical Equipment Co., Ltd. was used for fiber collection, and the energy consumption was 0.02 MJ h -1 . A yarn twist meter (Y331A, rated power≤25 W) from Changzhou Yifangyi Spinning Instrument Co., Ltd. was used for fiber twisting, and the energy consumption was 0.09 MJ h -1 . A small automatic roll-to-roll machine (MSK-HRP-04-RD, maximum power=20 W) from Hefei Kejing Material Technology Co., Ltd.
was used for fiber rolling, and the energy consumption was 0.07 MJ h -1 ( Supplementary Fig.   24).
Based on the energy consumption of CNT synthesis and post-processing of CNT fibers, the energy consumption per unit length of CNT fibers is 0.12 MJ, which is significantly lower than that of carbon fibers per unit length (>0.22 MJ) ( Supplementary Fig. 24). Therefore, we claimed that our process for preparing CNT fibers has a low energy consumption. Weichuang Electromechanical Equipment Co., Ltd. was used for fiber collection, and the energy consumption was 0.02 MJ h -1 . A yarn twist meter (Y331A, rated power≤25 W) from Changzhou Yifangyi Spinning Instrument Co., Ltd. was used for fiber twisting, and the energy consumption was 0.09 MJ h -1 . A small automatic roll-to-roll machine (MSK-HRP-04-RD, maximum power=20 W) from Hefei Kejing Material Technology Co., Ltd. was used for fiber rolling, and the energy consumption was 0.07 MJ h -1 . Figure 24. Comparison of energy consumption between lignin-based CNT fibers prepared by FCCVD method in our work and lignin-based carbon fibers prepared by conventional spinning method.

Comment 7: How was the porosity of the twisted and rolled fibers determined?
Response: We thank the reviewer for the comments.
The porosity of CNT fibers is calculated based on the density of CNT fibers and pure CNT materials. The densities of the TCFs and RCFs are 0.64 g cm -3 and 1.49 g cm -3 , respectively.
The density of pure CNT materials is 2.1 g cm -3 1 . According to the following equation, the porosities of TCFs and RCFs are calculated as 69.5% and 29%, respectively.
Corresponding changes: Page 22 in Manuscript: The density of the CNT fibers is calculated based on their mass and volume. The porosity (φ) of the CNT fibers is calculated according to the following equation: (%) =

Reviewer #2
Lignin is a bulk by-product of paper industry, whereas has not been availably used so far. In this work, high-performance CNT fibers were prepared by using industrial lignin as the carbon source under a highly continuous production speed. This paper was well written and the data was sufficient. This work would provide a new idea for the high-value utilization of industrial lignin or other biomass-based materials. However, some parts within the manuscript still needed to be improved. I think that it could be acceptable in Nature Communications after addressing the following comments: Response: We thank the reviewer for the positive comments. Response: We thank the reviewer for the comments.
According to literature research, the existence of C-C bonds between aromatic rings of lignin has been proved in some studies 1,2 . However, according to our NMR results, we did not find C-C bonds between aromatic rings of the raw lignin, which may be attributed to the low content of C-C bonds in the lignin we used.

Response:
We thank the reviewer for the comments.
CO mainly comes from the cleavage of ether bonds in the side chains and between the aromatic rings in lignin, as well as the secondary decomposition of some volatiles. CO2 mainly comes from the cleavage and reformation of reactive functional groups (such as carbonyl and carboxyl groups) in the side chains. CH4 is derived from the side chain cleavage and demethylation of methoxy groups on the benzene rings 1 . H2O is mainly produced by the hydroxyl groups on the aliphatic side chains of lignin 2 . The formation of H2 can be attributed to the rearrangement of broken bonds in the aromatic rings 3 .
CO is an effective carbon source for the synthesis of CNTs 4 , and its content is significantly higher than that of CH4 and CO2. The high yield of CO in lignin pyrolysis products can be attributed to its wide range of sources, including cleavage of the ether bonds in the side chains, cleavage of the ether bonds between the aromatic rings and the secondary decomposition of some volatiles.

Corresponding changes:
Page 7 in Manuscript: CO mainly comes from the cleavage of ether bonds in the side chains and between the aromatic rings in lignin, as well as the secondary decomposition of some volatiles. CO2 mainly comes from the cleavage and reformation of reactive functional groups (such as carbonyl and carboxyl groups) in the side chains. CH4 is derived from the side chain cleavage and demethylation of methoxy groups on the benzene rings 35 . H2O is mainly produced by the hydroxyl groups on the aliphatic side chains of lignin 36 . The formation of H2 can be attributed to the rearrangement of broken bonds in the aromatic rings 37 . CO is an effective carbon source for the synthesis of CNTs 38 , and its content is significantly higher than that of CH4 and CO2 due to the wide range of sources.

Response:
We thank the reviewer for the comments.
The purity of the generated CNTs was calculated based on the thermogravimetric analysis data of CNT fibers (Fig. 3h). The original CNT fibers are mainly composed of C and Fe elements (Figs. 3l-n). In thermogravimetric analysis, C was completely removed from the sample, and only Fe was remained. In fact, Fe in the sample was converted to Fe2O3 when heated at high temperature in air 1 , so the residue of the sample in Fig. 3h was Fe2O3. We recalculated the Fe content in the sample by removing oxygen. The actual Fe content (impurity content) in the sample is 17.3%. Therefore, the CNT mass fraction in the sample should be 82.7%.

Page 9 in Manuscript:
TGA result shows that the mass fraction of the CNTs in the aggregates is 82.7% (Fig. 3h), which is similar to the purity of the CNTs prepared by the same method 45 . Note that Fe in the sample was converted to Fe2O3 when heated at high temperature in air, so the removal of oxygen in Fe2O3 is required to calculate the impurity content 46 . In addition, based on the carbon content (61.9%) and feeding rate (4.8 mg min -1 ) of lignin as well as the preparation rate (1.46 mg min -1 ) and purity (82.7%) of the CNT aggregates, the yield of the CNTs is about 40.6%.

Comment 4:
The authors have mentioned that the rolling method can make fibers denser than the twisting method. The author should explain the corresponding mechanism in the text.

Response:
We thank the reviewer for the comments.
The rolling method can apply greater stress to the CNT fibers, and the CNTs in the fibers are closely arranged, resulting in greater fiber density (the density of RCFs is 1.49 g cm -3 ). As the friction between CNTs in the RCFs increases, and the slippage between CNTs becomes more difficult, thus significantly improving the mechanical properties of the fibers. However, for twisting method, too much twisting force will cause fiber fracture. Therefore, compared with rolling method, the force exerted by twisting on CNT fibers is smaller, and the resultant CNT fibers are loosely stacked with a density of only 0.64 g cm -3 , so the mechanical properties of the TCFs are also poor.
The corresponding mechanism about the rolling method can make fibers denser than the twisting method have been added in the revised manuscript and highlighted in blue.

Corresponding changes:
Page 14 in Manuscript: Compared with twisting, CNT fibers prepared by rolling (rolled CNT fibers, RCFs) have a denser structure ( Fig. 4d and Supplementary Fig. 19e) and improved fiber orientation ( Supplementary Fig. 19f) due to the greater stress applied to the CNT fibers. Response: We thank the reviewer for the comments.
We investigated the effect of lignin concentration on the morphology of CNTs. The results show that single-walled carbon nanotubes (SWNT) and double-walled carbon nanotubes (DWNT) can be obtained when the lignin concentration is lower than 0.8 mg mL -1 ( Supplementary Fig. 17). The acquisition of SWNT and DWNT can be attributed to the reduced amount of carbon deposited on the surface of the Fe catalyst. The existence of radial breathing mode (RBM) stretching vibration peak (100-300 cm -1 ) in Raman spectrum also proved the synthesis of SWNT (Supplementary Figs. 17d-e). When the lignin concentration is higher than 5.5 mg mL -1 , the excessive lignin will cause too many wall layers of CNTs, and a large number of carbon nanorods and amorphous carbon spheres also can be formed (Supplementary Figs. 17j-l).
We also studied the effect of injection rate of lignin solution (1-10 mL min -1 ) on the CNT fiber preparation. When the injection rate is lower than 1.5 mL min -1 , it's difficult to observe solid formation in the tubular furnace. When the injection rate is in the range of 1.5-2.5 mL min -1 , a small amount of CNT aerogels can be synthesized, but CNT fibers cannot be continuously prepared. The optimal injection rate range is 2.5-4.5 mL min -1 , in which CNT fibers can be continuously prepared. When the injection rate is higher than 4.5 mL min -1 , it's easy for the lignin solution to form aggregation and spray flame, which makes CNT fiber preparation unstable. Page 13 in Manuscript:

Supplementary
The effect of injection rate of lignin solution (1-10 mL min -1 ) on the CNT fiber preparation was further studied. When the injection rate is lower than 1.5 mL min -1 , it's difficult to observe solid formation in the tubular furnace. When the injection rate is in the range of 1.5-2.5 mL min -1 , a small amount of CNT aerogels can be synthesized, but CNT fibers cannot be continuously prepared. The optimal injection rate range is 2.5-4.5 mL min -1 , in which CNT fibers can be continuously prepared. When the injection rate is higher than 4.5 mL min -1 , it's easy for the lignin solution to form aggregation and spray flame, which makes CNT fiber preparation unstable.  Fig. 18a). These low-concentration lignin solutions can be injected directly into the tubular furnace for CNT synthesis. When the solution concentration increases to more than 1.5 mg mL -1 , lignin cannot be completely dissolved. After standing for 12 h at room temperature, lignin precipitates from the solutions obtained by magnetic stirring (Supplementary Fig. 18b). The amount of lignin precipitation depends on the solution concentration, and the higher the concentration, the more lignin is precipitated. For these high-concentration lignin solutions, they should be continuously oscillated to keep them in a uniform dispersion state during the process of CNT synthesis ( Supplementary Fig. 18c). Corresponding changes:

Supplementary
Page 13 in Manuscript: In the process of CNT synthesis, the way lignin solution is injected into the tubular furnace depends on the solution concentration. When the concentration of lignin solutions is less than 1.5 mg mL -1 , lignin can be completely dissolved in methanol. No lignin precipitate can be found in these solutions after standing at room temperature for 12 h (Supplementary Fig. 18a).
These low-concentration lignin solutions can be injected directly into the tubular furnace for CNT synthesis. When the solution concentration increases to more than 1.5 mg mL -1 , lignin cannot be completely dissolved. After standing for 12 h at room temperature, lignin precipitates from the solutions obtained by magnetic stirring (Supplementary Fig. 18b). The amount of lignin precipitation depends on the solution concentration, and the higher the concentration, the more lignin is precipitated. For these high-concentration lignin solutions, they should be continuously oscillated to keep them in a uniform dispersion state during the process of CNT synthesis ( Supplementary Fig. 18c).  Previous studies have shown that the length-diameter ratio (that is gauge length) of CNT fibers has little influence on their mechanical strength 1 . Therefore, a moderate length-diameter ratio of 10 mm was selected for the mechanical property determination of the CNT fibers in our study. Note that a gauge length of 10 mm is often used to test the mechanical properties of CNT fibers 2,3,4 .
Corresponding changes: Page 22 in Manuscript: The mechanical properties of the CNT fibers were determined using a universal tensile testing machine (YG-004, Dahua Electronic, China) and the gauge length was set as 10 mm.

2) Mass per unit length
The mass per unit length is calculated based on the mass and length of the CNT fibers, and the value for RCFs is 0.73 mg m -1 .

4) Elongation at break
According to the tensile stress-strain curves of TCFs and RCFs, their elongation at break are 6.12±0.43% and 5.62±0.18%, respectively.

Corresponding changes:
Page 14 in Manuscript: As can be seen from the cross-section of the TCFs, the inside of the fibers is not dense enough ( Supplementary Figs. 19c and 19d), which results in a low density of 0.64 g cm -3 ( Supplementary Figs. 20a-b).

Page 14 in Manuscript:
Compared with twisting, CNT fibers prepared by rolling (rolled CNT fibers, RCFs) have a denser structure ( Fig. 4d and Supplementary Fig. 19e) and improved fiber orientation ( Supplementary Fig. 19f) . 4g). RCFs have a denser structure and a more oriented structure compared to TCFs, which results in higher friction and more difficult slippage between CNTs in the fibers, thus achieving significantly better mechanical properties. The elongation at break of TCFs and RCFs are 6.12±0.43% and 5.62±0.18%, respectively. Although TCFs and RCFs have similar elongation at break, RCFs exhibit significantly higher fracture work due to their significantly higher mechanical strength.

Page 22 in Manuscript:
The mass per unit length is calculated based on the mass and length of the CNT fibers, and the value for RCFs is 0.73 mg m -1 .

Comment 9:
In the synthesis process, the author used a large number of catalysts based on the quality of raw lignin. How to control production cost? How are these catalysts disposed after the reaction?

Response:
We thank the reviewer for the comments.
Suitable catalyst concentration is very important for the continuous preparation of CNT fibers. In our work, we use ferrocene as the catalyst, and the concentration of ferrocene in the lignin solutions is 0.005 g mL -1 , lower than that used in many literatures for the preparation of CNTs by similar methods (Supplementary Table 9).
In order to further control the production cost of CNT fibers, the following aspects can be Corresponding changes: Page 19 in Manuscript: Although the method for improving the mechanical properties of CNT fibers by acid treatment and heat treatment have been reported 45, 83 , the additional processes inevitably increase the cost of fiber manufacturing and reduce the productivity, and are not conducive to the continuous preparation of CNT fibers.
Suitable catalyst concentration is very important for the continuous preparation of CNT fibers. In our work, we use ferrocene as the catalyst, and the concentration of ferrocene in the lignin solutions is 0.005 g mL -1 , lower than that used in many literatures for the preparation of CNTs by similar methods (Supplementary Table 9). In addition, the amount of catalyst is also very important to control the production cost of CNT fibers. In order to further control the production cost of CNT fibers, the following aspects can be considered:

Comment 10: The author only compared the literatures of the biomass-derived carbon fiber materials. Comparisons with CNTs fibers generated by classical FCCVD methods should also
be considered to support the superiority of this work.

Response:
We thank the reviewer for the comments.
According to the suggestion of the reviewer, we compared the mechanical strength and electrical conductivity of our CNT fibers with the reported biomass-derived carbon fibers, array CNT fibers, CNT fibers from FCCVD and wet-spun CNT fibers, as well as commercial carbon fibers and common metal materials.

Corresponding changes:
Page 15 in Manuscript:

Page 17 in Manuscript:
We also proved that our CNT fibers have high electrical conductivity, and the electrical conductivity of the CNT fibers with a density of 1.49 g cm -3 is as high as (6.03±0.25)×10 5 S m -1 , which is similar to that of the CNT fibers with similar structures prepared by the similar method 67 . The electrical conductivity of our CNT fibers is higher than that of almost all reported biomass-derived carbon fibers and array CNT fibers as well as most commercial carbon fibers (Fig. 4j). It is worth noting that the electrical conductivity of our CNT fibers is lower than that of most wet-spun CNT fibers, which may be due to the higher purity and crystallinity of the CNTs used for wet-spinning as well as the higher density of the resultant CNT fibers (Fig. 4j and Supplementary Table 7).
Although the mechanical strength of the prepared CNT fibers is not yet comparable to that of commercial carbon fibers, it is higher than or similar to that of most reported biomassderived carbon fibers, array CNT fibers, CNT fibers from FCCVD and wet-spun CNT fibers, as well as all common metal materials ( Fig. 4j and Supplementary

Page 17 in Manuscript:
We also proved that our CNT fibers have high electrical conductivity, and the electrical conductivity of the CNT fibers with a density of 1.49 g cm -3 is as high as (6.03±0.25)×10 5 S m -1 , which is similar to that of the CNT fibers with similar structures prepared by the similar method 67 . The electrical conductivity of our CNT fibers is higher than that of almost all reported biomass-derived carbon fibers and array CNT fibers as well as most commercial carbon fibers (Fig. 4j). It is worth noting that the electrical conductivity of our CNT fibers is lower than that of most wet-spun CNT fibers, which may be due to the higher purity and crystallinity of the CNTs used for wet-spinning as well as the higher density of the resultant CNT fibers (Fig. 4j and Supplementary Table 7).
Although the mechanical strength of the prepared CNT fibers is not yet comparable to that of commercial carbon fibers, it is higher than or similar to that of most reported biomassderived carbon fibers, array CNT fibers, CNT fibers from FCCVD and wet-spun CNT fibers, as well as all common metal materials ( Fig. 4j and Supplementary Table 7). It should be emphasized that the mechanical strength of our CNT fibers exceeds that of most CNT fibers prepared with fine chemicals (such as alkanes and aromatic hydrocarbons) as carbon sources.
Taken together, our lignin-derived CNT fibers show the unprecedented integration of high tensile strength, thermal conductivity, and electrical conductivity, as well as continuous preparation process.

Page 18 in Manuscript:
Compared with the preparation of CNT fibers using fine chemicals as raw materials, the preparation efficiency of our method is lower because it takes a certain amount of time for lignin to decompose into small molecules (Supplementary Table 8). However, the production of traditional lignin-based carbon fibers involves spinning and multi-step heat treatment. It takes at least 90 minutes to get lignin-based carbon fibers, and the fiber preparation rate is only 20-35 m h -1 81 .
Page 9 in Supplementary Information:

Response:
We thank the reviewer for the comments.
The original CNT fibers are mainly composed of C and Fe elements (Figs. 3l-n). In thermogravimetric analysis, C was completely removed from the sample, and only Fe was remained. In fact, Fe in the sample was converted to Fe2O3 when heated at high temperature in air 1 , so the residue of the sample in Fig. 3h was Fe2O3. We recalculated the Fe content in the sample by removing oxygen. The actual Fe content (impurity content) in the sample is 17.3%.
Therefore, the CNT mass fraction in the sample should be 82.7%. In addition, the CNTs prepared using our method have an IG/ID of 3.84, indicating that they are well crystallized. conductivity of the CNT fibers with a density of 1.49 g cm -3 is 6.03×10 5 S m -1 , which is similar to that of the CNT fibers with similar structures prepared by the similar method (Table R1) Our CNT films with a density of 0.82 g cm -3 exhibit a thermal conductivity of 33.21±0.76 W m -1 K -1 , which is close to that of the CNT films with similar characteristics prepared by the similar method (Table R2)   TGA result shows that the mass fraction of the CNTs in the aggregates is 82.7% (Fig. 3h), which is similar to the purity of the CNTs prepared by the same method 45 . Note that Fe in the sample was converted to Fe2O3 when heated at high temperature in air, so the removal of oxygen in Fe2O3 is required to calculate the impurity content 46 . In addition, based on the carbon content (61.9%) and feeding rate (4.8 mg min -1 ) of lignin as well as the preparation rate (1.46 mg min -1 ) and purity (82.7%) of the CNT aggregates, the yield of the CNTs is about 40.6%.

Page 16 in Manuscript:
In addition to excellent mechanical properties, the CNT films with a density of 0.82 g cm -3 exhibit high thermal conductivity of 33.21±0.76 W m -1 K -1 (Supplementary Fig. 23).
Compared to biomass-derived carbon materials (0.06-24 W m -1 K -1 ), our CNT films possess higher thermal conductivity, comparable to that of the CNT films with similar characteristics prepared by the similar method (20.91-458.58 W m -1 K -1 ) as well as some common metals (30-500 W m -1 K -1 ) ( Fig. 4i and Supplementary Table 6). Considering that the CNT films have significantly lower density (0.82 g cm -3 ) than common metals (2.7-10.49 g cm -3 ), they can be used in some fields that require lightweight thermal conductive materials.

Page 17 in Manuscript:
We also proved that our CNT fibers have high electrical conductivity, and the electrical conductivity of the CNT fibers with a density of 1.49 g cm -3 is as high as (6.03±0.25)×10 5 S m -1 , which is similar to that of the CNT fibers with similar structures prepared by the similar method 67 . The electrical conductivity of our CNT fibers is higher than that of almost all reported biomass-derived carbon fibers and array CNT fibers as well as most commercial carbon fibers (Fig. 4j). It is worth noting that the electrical conductivity of our CNT fibers is lower than that of most wet-spun CNT fibers, which may be due to the higher purity and crystallinity of the CNTs used for wet-spinning as well as the higher density of the resultant CNT fibers (Fig. 4j and Supplementary Table 7). Response: We thank the reviewer for the comments.

Supplementary
The electrical conductivity of the CNT fibers was measured by two-probe method. The distance between the two probes was set at 1 cm. The resistance of the CNT fibers was measured by a multimeter, as shown in Fig. R3a. The electrical conductivity (σ, S m -1 ) is calculated by the following equation: σ = × , where L is the distance between the two probes (L=1 cm), R is the resistance of the CNT fibers (Ω), and S is the cross-sectional area (m 2 ). The samples used for electrical conductivity test are RCFs, whose cross-section is rectangular (Figs. R3b-d). Five conductivity values are obtained and their average is presented, that is (6.03±0.25) ×10 5 S m -1 . We also proved that our CNT fibers have high electrical conductivity, and the electrical conductivity of the CNT fibers with a density of 1.49 g cm -3 is as high as (6.03±0.25)×10 5 S m -1 , which is similar to that of the CNT fibers with similar structures prepared by the similar method 67 . The electrical conductivity of our CNT fibers is higher than that of almost all reported biomass-derived carbon fibers and array CNT fibers as well as most commercial carbon fibers (Fig. 4j). It is worth noting that the electrical conductivity of our CNT fibers is lower than that of most wet-spun CNT fibers, which may be due to the higher purity and crystallinity of the CNTs used for wet-spinning as well as the higher density of the resultant CNT fibers (Fig. 4j and Supplementary Table 7).

Page 22 in Manuscript:
The electrical conductivity of the CNT fibers was measured by two-probe method. The distance between the two probes was set at 1 cm. The resistance of the CNT fibers was measured by a Digit Graphical Touchscreen Digital Multimeter (DMM6500 6½, Keithley, USA). The electrical conductivity (σ, S m -1 ) is calculated by the following equation: where L is the distance between the two probes (L=1 cm), R is the resistance of the CNT fibers (Ω), and S is the cross-sectional area (m 2 ). RCFs with a rectangular cross-section were used for the electrical conductivity determination. Five conductivity values were obtained and their average was presented. Response: We thank the reviewer for the comments.
According to the suggestion of the reviewer, error analysis has been conducted in the revised manuscript.  (Fig. 4h).

Page 17 in Manuscript:
We also proved that our CNT fibers have high electrical conductivity, and the electrical conductivity of the CNT fibers with a density of 1.49 g cm -3 is as high as (6.03±0.25)×10 5 S m -1 , which is similar to that of the CNT fibers with similar structures prepared by the similar method 67 .

Comment 4: Another problem with this article is that the lignin-derived material does not
show benefits compared to CNTs made from synthetic precursors in terms of properties.

Response:
We thank the reviewer for the comments.
First of all, in terms of mechanical property, our CNT fibers exhibit a tensile strength of 1.33 GPa, which is higher than or similar to that of most reported biomass-derived carbon fibers, array CNT fibers, CNT fibers from FCCVD and wet-spun CNT fibers, as well as all common metal materials (Fig. 4j). It should be emphasized that the mechanical strength of our CNT fibers exceeds that of most CNT fibers prepared with fine chemicals (such as alkanes and aromatic hydrocarbons) as carbon sources 1,2,3 .
In terms of electrical conductivity, our CNT fibers have an electrical conductivity of 1.19 ×10 5 S m -1 , which is similar to that of the CNT fibers with similar structures prepared by the similar method 4 . The electrical conductivity of our CNT fibers is higher than that of almost all reported biomass-derived carbon fibers and array CNT fibers as well as most commercial carbon fibers (Fig. 4j). The electrical conductivity of our CNT fibers is lower than that of the wet-spun CNT fibers, which may be due to the higher purity and crystallinity of the CNTs used for wet-spinning as well as the higher density of the resultant CNT fibers (Fig. 4j).
In addition to excellent mechanical and electrical properties, our CNT films with a density of 0.82 g cm -3 exhibit high thermal conductivity of 33.21 W m -1 K -1 . Compared to biomassderived carbon materials (0.06-24 W m -1 K -1 ), our CNT films possess higher thermal conductivity, comparable to that of the CNT films with similar characteristics prepared by the similar method (20.91-458.58 W m -1 K -1 ) as well as some common metals (30-500 W m -1 K -1 ) ( Fig. 4i). It should be emphasized that the thermal conductivity of CNT films increases gradually with increasing density 5 . Considering that our CNT films have significantly lower density (0.82 g cm -3 ) than common metals (2.7-10.49 g cm -3 ), they can be used in some fields that require lightweight thermal conductive materials.
In addition to material properties, cost and energy consumption also need to be considered.
Lignin was used as the carbon source to prepare CNT fibers in our work, and it is derived from the by-products of the pulp and paper industry, and the cost is negligible. From the point of view of energy consumption, the energy consumption of our CNT fibers is estimated to be about 0.12 MJ m -1 , which is significantly lower than that of lignin-based carbon fibers prepared by traditional methods (0.22-0.67 MJ m -1 ) (Supplementary Fig. 26).

Fig. 4 Preparation and properties of lignin-derived CNT fibers. Preparation diagrams of (a)
TCFs and (b) RCFs. SEM images of (c) TCFs and (d) RCFs. Polarised Raman spectra of (e) TCFs Last but not least, we need to reemphasize that our work achieved for the first time the continuous preparation of CNT fibers from waste lignin as carbon source. The preparation rate of the CNT fibers is up to 120 m h -1 , which is significantly higher than that of lignin-based carbon fibers prepared by traditional methods (20-35 m h -1 ) (Supplementary Fig. 26). The high preparation efficiency and low energy consumption combined with low lignin pretreatment requirements make our method very promising for large-scale production of lignin-based CNT fibers.
Supplementary Figure 26. Comparison of energy consumption between lignin-based CNT fibers prepared by FCCVD method in our work and lignin-based carbon fibers prepared by conventional spinning method 63,64 .